1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system which varies its magnification around unity and is particularly adapted for use in a copying machine or a process camera as an imaging lens, or in various optical systems as a relay lens.
2. Description of the Prior Art
In the copying machine and the process camera, the area of the object and the area of the focused image are generally very large in comparison with ordinary photography. Therefore, the diameter and the length of the lens system used therein are comparatively large in order to obtain a necessary angle of view and a necessary focal length. From the viewpoint of manufacturing and the structure of the machine, the lens system used therein is desired to be made as compact as possible. Further, in the copying machine and the process camera, the lens is required to be completely free from aberrations. Particularly, the distortion of the image should strictly be eliminated in this kind of lens.
Therefore, the lens system used in the copying machine or in the process camera is generally a symmetric type lens which is theoretically free from distortion at a magnification of unity. In this case, in order to change the magnification of the optical system without changing the focal length of the lens, at least two out of the three elements that are the object plane, the image plane and the lens must be moved as is well known in the art. In practical use, however, it is very troublesome and undesirable to move two of these elements. Even if the two elements which are to be moved together are interlocked with each other by means of a motion transmitting mechanism, the structure of the copying machine or the like would be very much complicated. Further, it is theoretically inevitable that the distance between the object and the image be elongated when the magnification is changed from unity. Besides, even when a symmetric type lens which is free from distortion when magnification is unity is used, there appears distortion when the magnification is varied from unity. Of course, when the magnification is greatly changed, other aberrations also appear and deteriorate the quality of the image obtained.
On the other hand, it has been known in the art to use attachment lenses in order to change the magnification of a focusing lens in a copying machine or the like without changing the positions of the object and the image. This method, however, is disadvantageous in that the magnification cannot be varied continuously and a complicated mechanism is required to substitute the attachment lenses every time when the magnification is to be changed. Further, in this method, if a wide magnification range is desired, a high power attachment lens is necessary, and thereby various aberrations increase in addition to the distortion significantly increasing.